Multiple KeyErrors leading to segfaults with custom properties
I might be mis-interpretting your question but, this works:

import bpy
import math

def update_rotate_cam(self, context):
    v = bpy.context.scene.rotCam
    bpy.data.objects['Camera'].rotation_euler.x = math.radians(v)

class SliderProperties(bpy.types.PropertyGroup):
    bpy.types.Scene.rotCam = bpy.props.FloatProperty(
        name="Rotate Camera",
        description="Rotate the camera with the slider",
        default=0,
        min=0,
        max=360,
        update=update_rotate_cam,
    )

class EditStuff(bpy.types.Panel):
    bl_label = "Edit"
    bl_space_type = "VIEW_3D"
    bl_region_type = "TOOLS"
    bl_context = "objectmode"

    def draw(self, context):
        layout = self.layout
        layout.prop(bpy.context.scene, "rotCam")

def register():
    bpy.utils.register_module(__name__)

def unregister():
    bpy.utils.unregister_module(__name__)

if __name__ == "__main__":
    register()


In python a safe way to access items using a key is often done this way:

# returns None if key isn't found 
v = some_collection.get(key_name) 

# or provide the fallback value if not found
v = some_collection.get(key_name, fallback)